1. Field of the Invention
The present invention relates to devices useful for equalizing fluid pressures. More particularly, the present invention relates to pressure equalization valves useful for equalizing fluid pressures between plural fluid sources.
2. Description of the Invention Background
As shown in FIG. 1, which is a diagram of a prior art material handling system, an airlock, such as an airlock 1, is a device used for the transfer, by gravity, of fly ash or other dry, free-flowing granular solids from one pressure zone to another. For example, in the system of FIG. 1, granular solids are collected in an overhead hopper 2 and are dropped into the airlock 1 in controlled volumes through operation of a pneumatically-operated valve 3. Each controlled volume of granular solids is then dropped, by means of another pneumatically-operated valve 4, into a conveying pipe 5 carrying a supply of pressurized conveying fluid, such as air. The air in the conveying pipe is generally compressed up to 20 psi, but may be compressed up to 25 psi.
Because the pressurized conveying fluid is normally at a much higher pressure than the near-atmospheric pressure inside the airlock 1, the pressure differential would slow down the movement of solids from the airlock 1 into the pipe 5 if it were not equalized. Accordingly, prior art material handling systems such as that shown in FIG. 1 often include a pressure equalization valve 6.
With reference to FIG. 1 and FIGS. 2-3, which illustrate two prior art equalization valves 6', 6", equalization valves 6', 6" each include a port 7', 7", respectively, in fluid communication with an airlock 1 via a fluid line 7, a port 8', 8", respectively, in fluid communication with a conveying pipe 5 at point 5a via a fluid line 8, and a port 9', 9", respectively, in fluid communication with a hopper 2 via a fluid line 9. Each equalization valve 6', 6" also includes valve gates 10', 10", respectively, operable by an actuator 11', 11", respectively, for alternately sealing the ports 8', 9' and 8", 9", respectively.
An equalization valve 6, 6', 6" normally operates as follows. Before a volume, of granular solids is dropped from the hopper 2 into the airlock 1, the valves 3, 4 are in a closed position. The actuator 11', 11", respectively, is in a position such that the port 8', 8", respectively, is sealed and the port 9', 9" is open. The hopper 2 and the airlock 1 are thus in fluid communication, and the pressure of the air in the hopper 2 above the solids and the pressure of the air in the airlock 1 are nearly equal and nearly at atmospheric pressure. The volume of granular solids is then dropped into the airlock 1 by opening and timed closing of the valve 3, and the actuator 11', 11", respectively, is then operated so that the port 8', 8", respectively, is open and the port 9', 9", respectively, is sealed. The conveying pipe 5 is thus in fluid communication with the airlock 1 at point 5a, and the pressure of the air in the airlock 1 thus nearly equalizes to the much higher pressure of the air in the conveying pipe 5 at point 5a. The pressures normally do not completely equalize because of the pressure drops inherent in the system. The entire pressure equalization process usually takes no more than 5-15 seconds, depending on the volume of the airlock 1.
The solids are then dropped into the conveying pipe 5 at point 5c by the opening of the valve 4, and are carried away through the pipe 5. It should be noted that the pressure of the air at the point 5c is much lower than at the point 5a (and thus in the airlock 1), because an orifice is interposed in the pipe 5 at point 5b intermediate points 5a and 5c. The resultant pressure drop between the airlock and point 5c encourages the flow of solids into the pipe 5. When the valve 4 is closed, some residual solids generally remain in the airlock 1. The actuator 11', 11", respectively, is then operated to seal the port 8', 8", respectively, and to open the port 9', 9", respectively, such that the airlock 1 and the hopper 2 are again in fluid communication and the pressure of the air in the airlock 1 can equalize with the near-atmospheric pressure of the air in the hopper 2. During each pressure equalization process, the velocity of the air passing through the equalization valve 6', 6" can reach almost sonic speeds, due to the extreme pressure differential between the air in the conveying pipe 5 and the airlock 1, and then the airlock 1 and the hopper 2. In addition, during the pressure equalization process between the airlock 1 and the hopper 2, the air flowing through the equalization valve 6', 6" normally entrains a significant amount of loose granular solids from the airlock 1, which air/solids mixture is normally very abrasive when flowing at high speeds.
Prior art pressure equalization valves 6', 6" have significant disadvantages. First, such valves 6', 6" tend to have very rapid wear rates, especially with respect to the valve ports 9', 9", the valve gates 10', 10" and the actuators 11', 11", respectively, which are cyclically subjected to blasts of granular articulates entrained in air streams moving at near-sonic speeds. It has been estimated that wear rates on such parts are proportional to v.sup.3, where v is equal to the velocity of the air-solids mixture. For some prior art equalization valves 6, it is not uncommon for the valves 6 to require replacement within three months of being put into service.
Further, pressure equalization valves such as the valves 6', 6" are designed such that, when an actuator 11', 11", respectively, is operated to move between port sealing positions, there is a period of time where both the port 8', 9' and the port 8", 9", respectively, are open, allowing pressure equalization between the airlock 1, the hopper 2 and the conveying pipe 5 all at the same time. In systems that are vacuum-operated, as opposed to the pressurized system shown in FIG. 1, this transient three-way equalization has been found to result in the suction of granular solids into the vacuum pumps, and thus increased maintenance of such pumps.
Pressure equalization valves of the types described above are typically associated with relative high maintenance costs. For example, valves used in connection with the transmission of particulate such as ash and the like typically suffer significant wear problems. Such particulate material tends to erode the gate and seats which, over time, can cause leaks to develop when the gate is closed. Often times, when such leaks develop, the system must be disabled to permit the valve to be accessed and or replaced. Such system downtime can lead to increased operating costs. Furthermore, due to their design, prior valves often must be substantially disassembled to gain access to the worn parts. In some valve designs, because the seats are machined into the housing, the valve housing must be replaced or repaired leading to increased maintenance costs and downtime.
In view of the above, it is a feature of the present invention to provide an improved pressure equalization valve.
It is another feature of the present invention to provide a pressure equalization valve that wears at a relatively slow rate.
A further feature of the present invention is to provide a pressure equalization valve that restrains unintended transient equalization of fluid sources.
Another feature of the present invention is to provide a pressure equalization valve having internal components that are relatively wear resistant.
Yet another feature of the present invention is to provide a pressure equalization valve having internal components that are relatively easy to service and replace.